Vehicle control systems are known which perform collision avoidance control, in which the vicinity of a traveling vehicle is scanned by a vehicle radar device, and when collision with an obstruction is predicted, the vehicle is accelerated/decelerated or a safety device is activated. As a vehicle radar device used in such a system, electronic scanning type radar devices are known. As disclosed in an example in Patent Reference 1, an electronic scanning type radar device uses a plurality of antennas to receive radar waves reflected by a target, and based on the phase difference of the received waves of the antennas, detects the arrival direction of the received waves, that is, the azimuth angle of the target.
FIG. 1A and FIG. 1B explain the principle of detection of the azimuth angle of a target by an electronic scanning type radar device. As shown in FIG. 1A and FIG. 1B, an electronic scanning type radar device 10 transmits a radar wave W1 with the radar device front face as the reference direction F, and receives the radar wave W1 reflected by the target T using two antennas 11, 12 as the received waves W21, W22. Here, the distance d between the antennas 11 and 12 is minute compared with the distance to the target T, so that the transmission paths of the received wave W21 received by the antenna 11 and of the received wave W22 received by the antenna 12 can be regarded as parallel. And, the arrival direction of the received waves W21 and W22, that is, the azimuth angle of the target T with respect to the reference direction F (azimuth angle 0°), is θ.
First, as shown in FIG. 1A, when the azimuth angle θ of the target T is 0°, that is, when the received wave pair W21, W22 arrive from the reference direction F, the pair of received waves W21, W22 arrive simultaneously at the antenna pair 11, 12. At this time, the pair of received waves W21, W22 have the same phase. That is, there is no phase difference between these received waves.
Next, as shown in FIG. 1B, when the azimuth angle θ of the target T is shifted to the left from the reference direction F, a difference in the traveling distance of the pair of received waves W21, W22 of Δd (=d·sin θ) occurs which is proportional to the antenna distance d. Hence the time of arrival of the received wave W22 at the antenna 12 is delayed from the time of arrival of the received wave W21 at antenna 11 by the amount equivalent to the difference in traveling distances Δd. At this time, if the wavelength of the received waves W21 and W22 is λ, then a phase difference φ(=Δd·2π/λ) occurs, corresponding to the delay time between the pair of received waves W21 and W22. This applies in cases in which the azimuth angle θ of the target T is shifted to the right of the reference direction as well.
In this way, from the phase difference φ between the pair of received waves W21 and W22, the azimuth angle θ of the target T can be determined from expression (1) below.θ=arcsin(λ·φ/(2π·d))  (1)
Here, if the phase difference φ between the received waves exceeds ±π, that is, if so-called phase wrapping occurs, then as indicated in expression (2) below, θ cannot be uniquely determined from the phase difference φ.θ=arcsin(λ·(φ±kπ)/(2π·d))(k=0,1,2, . . . )  (2)
Hence the range over which the azimuth angle θ is uniquely determined from the phase difference φ is indicated by the following expression (3), taking k=1 in expression (2).−arcsin(λ·φ/(2d))arcsin(λ·φ/(2d))  (3)
The relation between the phase difference φ and azimuth angle θ in expression (3) is represented by the straight line L in FIG. 2A. That is, in the range ±arcsin(λ·φ/(2d)), the azimuth angle θ1 is uniquely determined from the phase difference φ1. In the following, the range of azimuth angles in which the azimuth angle θ is uniquely determined from the phase difference φ in this way is called the phase wrapping interval (A1), and the azimuth angle −arcsin(λ·φ)/(2d)) and +arcsin(λ·φ/(2d)) at which phase wrapping occurs are represented by θa and θb respectively.
Hence in ranges outside the phase wrapping interval A1, for example the phase difference φ2 corresponds to a plurality of azimuth angles, that is, to an azimuth angle θ21 outside the phase wrapping interval A1, and an azimuth angle θ22 within the phase wrapping interval A1.
FIG. 2B shows the azimuth angle of a target T in the scanning plane of the radar device 10. If the phase difference of a pair of received waves from the target existing at azimuth angle θ21 is φ2, then according to the correspondence relation of FIG. 2A, the azimuth angles θ21 and θ22 are obtained. Operations of a vehicle are controlled with respect to targets existing in the azimuth angle range in which dangers due to collision or other factors are great. The range of these azimuth angles is taken to be a phase wrapping interval A1. Then, assuming that a target T actually existing at the azimuth angle θ21 also exists at the azimuth angle θ22, by making an erroneous detection (indicated by the dotted line in the figure), vehicle operation is controlled with respect to a target which does not actually exist. As a result, a hindrance to safety occurs.
Hence in order to prevent hindrances with erroneous detection, various methods have been proposed in the past. One example is a method in which, after determining the azimuth angle from the phase difference between a pair of received waves as described above, radar waves are again transmitted and received, and the levels of the newly received waves are used to make a true/false judgment of the azimuth angle previously determined. Specifically, in this method, for an azimuth angle θ22 detected within the phase wrapping interval A1, judgment is performed as to whether a target actually exists (true) or does not (false) at the azimuth angle in question. FIG. 3A and FIG. 3B explain this method.
In FIG. 3A, the azimuth angle is plotted along the horizontal axis, and the vertical axis plots the level of waves received by an antenna 11 obtained by reflection from a target when the same target, such as for example a passenger vehicle or other small vehicle exists at all azimuth angles. The received wave level is maximum in the reference direction F, that is, for a target existing at azimuth angle 0°. And, as the azimuth angle shifts from 0°, the level of the received wave obtained from the target existing at that azimuth angle declines. That is, the level of the wave received by the antenna 11 is maximum when the arrival direction of the received wave is azimuth angle 0°, and drops as the arrival angle approaches azimuth angles of ±90°. In other words, an antenna pattern P1 having directivity at azimuth angle 0° is described. This applies also to an antenna 12 with the same configuration as the antenna 11.
Next, the phase wrapping interval A1 shown in FIG. 2A is applied to this antenna pattern P1, and the reception level T1 obtained at the end portions of the interval, that is, at the azimuth angles θa and θb, is taken to be the threshold value for true/false judgment. When the level of a wave received by the antenna 11 exceeds the threshold value T1, the received wave is from a target existing within the phase wrapping interval A1. Hence in this case it is possible to infer the existence of an actual target within the phase wrapping interval A1. Conversely, when the level of a received wave does not exceed the threshold value T1, the received wave is from a target existing outside the phase wrapping interval A1, and so it can be inferred that the target does not exist in the phase wrapping interval A1.
Using this fact, the radar device 10 first determines the azimuth angle θ22 indicated in FIG. 2B, and then performs radar wave transmission and reception, and compares the level of the wave received by the antenna 11 with the threshold value T1. If the level of the received wave is lower than the threshold value T1, that is, if the target does not exist within the phase wrapping interval A1, the azimuth angle θ22 thus determined can be inferred to be a false azimuth angle arising from phase wrapping. Hence the radar device 10 judges this to be false. In this case, the detection result is not output to the vehicle control device.
When the level is equal to or greater than T1, a target is judged to actually exist at the detected azimuth angle θ22. In this case, in contrast with the case of FIG. 2B, a target actually exists within the phase wrapping interval A1, and the azimuth angle θ22 has been determined from the phase difference in the pair of waves received from the target. Hence the azimuth angle θ22 is judged to be the correct azimuth angle, and the detection result is output.
By making a true/false judgment of a detected azimuth angle based on received wave levels in this way, a radar device 10 can prevent vehicle control based on erroneous detection.    Patent Reference 1: Japanese Patent Application Laid-open No. 2005-3393
However, the levels of waves received at the same azimuth angle differ greatly with the magnitude of the reflection cross-sectional area of the target, and are lower for small reflection cross-sectional areas. In contrast with the antenna pattern P1 obtained from a small vehicle shown in FIG. 3A, FIG. 3B shows the antenna pattern P2 obtained from a truck or other large vehicle, as an example of a case in which the reflection cross-sectional area is large. That is, by displacing the antenna pattern P1 upward, the antenna pattern P2 is obtained.
When making a true/false judgment of the azimuth angle θ22 shown in FIG. 2B, the level of the wave received from the target existing at azimuth angle θ21 is equal to or greater than the threshold value T1. Hence because the received wave level is equal to or greater than the threshold value, the radar device 10 judges the azimuth angle θ22 to be correct. Hence the vehicle operation is controlled with respect to a target which does not actually exist at the azimuth angle θ22, and a hindrance to safety occurs.